1. Field of the Invention
The invention relates generally to semiconductor power devices. More particularly, this invention relates to new configurations and methods for manufacturing improved device structures for insulated gate bipolar transistors (IGBT) with dual gates to provide trench shield and further providing buried floating shield ring under the trench to improve the UIS ruggedness of the IGBT device.
2. Description of the Prior Art
Conventional technologies to configure and manufacture insulated gate bipolar transistor (IGBT) devices are still confronted with difficulties and limitations to further improve the performances due to different tradeoffs. In IGBT devices, there is a tradeoff between the conduction loss VCE,sat (which depends upon the collector to emitter saturation voltage at rated current VCE,sat) and turn-off switching losses, Eoff. More carrier injection while the device is on improves the conductivity of the device, thus reducing conduction loss, but more carrier injection would also cause higher Eoff, because of the energy dissipated when clearing out the injected carriers during turn-off. FIG. 1D is a graph showing the trade-off between VCE,sat and Eoff. It can be observed that the curve for a superior IGBT structure will be shifted closer to the origin, corresponding to lower losses.
In addition a trade-off also exists between the IGBT VCE,sat (conduction loss) and the IGBT's short circuit ruggedness, which in turn depends upon its saturation current Jsat. A high Jsat will result in a lot of energy dissipated in the device during short circuit, which would quickly damage the IGBT device. A lower Jsat will reduce the amount of energy dissipated, allowing the IGBT device to withstand the short circuit for a longer period of time without permanent damage; however, a lower Jsat also results in higher conductivity loss VCE,sat.
FIG. 1A shows the cross section of a conventional planar gate insulated gate bipolar transistor (IGBT). The IGBT is a semiconductor power device that combines a metal oxide-semiconductor (MOS) gate control with a bipolar current flow mechanism. The functional features of both a metal-oxide-semiconductor field effect transistor (MOSFET) and a bipolar junction transistor (BJT) are combined in an IGBT. Performance features of IGBT are designed to achieve a higher current density than the MOSFETs and faster and more efficient switching characteristics and better control than the BJTs. The drift region can be lightly doped for improved blocking abilities. However the device can still have good conductivity because the lightly doped drift region undergoes high level carrier injection from the bottom P collector region resulting in its conductivity modulation. For these reasons, IGBT devices are often implemented for high power (>10 kW), low to medium frequency (up to 30 kHz) applications. The planar IGBT device shown in FIG. 1A has a simple top side structure and is easy to fabricate. However, the planar gate IGBT as shown in FIG. 1A has high VCE,sat due to poor conductivity modulation near the top side and in addition high JFET resistance due to pinching from neighboring body regions. FIG. 1B is a cross sectional view of another conventional IGBT device that has a trench gate. The trench gate IGBT has the advantages of eliminating the JFET resistance and also has an enhanced top side carrier injection. An accumulation layer can form under the trench gate to improve carrier injection. However, the trench IGBT device as shown has a high Crss capacitance due to capacitance between the trench gate (at gate voltage) and the substrate and drift region below (at drain voltage). The high Crss of this IGBT device slows down the device switching speed and also leads to higher switching energy losses. FIG. 1C is a cross sectional view of another conventional IGBT device. There is a more heavily doped N layer disposed below the channel region, at the top of the lightly doped drift region to further enhance the carrier injection on the topside. However, such device has a lower breakdown voltage due to the heavily doped layer and has a further worsened Crss due to the heavily doped N-layer.
For the above reasons, there is a need to provide a new IGBT configuration to reduce the turn-on and turn off energy Eon losses and Eoff losses for improvement of the operation efficiency. Furthermore, it is desirable the new IGBT with the improved configuration can reduce the Crss, increase the breakdown voltage, improve VCE,sat and to increase the cell pitch to lower the Jsat such that the above discussed limitations and difficulties can be resolved.